Water and Wastewater

Some of the readers may have come across one or more of the words ‘water treatment plant’ (WTP), ‘sewage treatment plant’ (STP) and ‘effluent treatment plant’ (ETP). A brief description of these three is given below. All these processes deal with treatment of water so that the levels of certain substances in it are reduced to acceptable levels based on what the water will be used for after treatment. While water treatment mainly deals with processing the water so that it becomes fit for specific uses, both sewage treatment and effluent treatment are done on used water or ‘wastewater’ to remove the contaminants added to it due to use so that it is fit for being released into other water bodies. Directly releasing untreated wastewater into natural water bodies leads to situations such as rivers foaming excessively and lakes catching fire.

Water treatment plant

Water treatment plants are used for treatment of ‘raw’ water before supplying it further for use by people. Water from sources such as rivers, lakes, dam reservoirs etc. is first supplied to a water treatment plant where it undergoes the required processing and then it is pumped for further use.

Raw water contains a number of undesirable things such as biological material, silt and other particles, microbes, some chemicals, hardness etc. All these things are to be reduced to a prescribed level to make this water fit for usage. This is done through a number of processes in a water treatment plant. Treatment requirements are different for municipal (human consumption) use and industrial use as there is a difference in the characteristics of the water required for these purposes.

Water treatment plants for municipal use often have an ‘aeration fountain’, where the water emerges like in a fountain and falls down in layers while getting exposed to the atmosphere. This causes some of the dissolved gases in the water to escape into the atmosphere, and also enables contact of atmospheric oxygen with the water. In the next stage, chemicals are added to help in coagulation and the water is agitated for thorough mixing. This is done in a large storage vessel where the solids coagulate and settle down. The water is then fed through filter beds where filtration takes place and more of the finer solid particles are removed. After filtration, the water is dosed with the necessary amount of chlorine and is then sent further into the distribution system.

Typically, only a small number of water treatment plants is installed for a system supplying a city or a cluster of closely-located villages or small settlements. The combined capacity of these plants can easily be in the range of several millions of litres per day. Some of these plants may be located at high elevation areas such as hills, from where the water is supplied further by gravity, or may have attached ‘clear water’ pump houses to pump the water further.

While municipal water supply requires removal of micro-organisms and other disease-causing pathogens as a priority, industrial use is more focused on reducing or eliminating mineral salts which may cause issues like scaling and corrosion of equipment. This pre-use treatment of water in industries is often called ‘demineralization’ and the treatment setup is commonly known as ‘DM plant’ in short. Ion exchange plants are often used, which contain layers of specific resins which are able to exchange the less favourable metal ions from dissolved salts for ions of other metals which cause much less damage. An example is the removal of calcium and magnesium ions from salts in hard water, which cause a layer to get deposited inside pipes and tubes and hinder free movement through them, with sodium.

Sewage treatment plant

Sewage water is the water that gets discarded along with human excretion and other waste materials such as soap etc. The concentration of biological waste, microbes etc. in this water should be reduced to prescribed levels before releasing it into other water streams for health and environmental reasons. ‘Biological Oxygen Demand’ (BOD) and ‘Chemical Oxygen Demand’ (COD) are measures of the level of dissolved oxygen in the water. High levels of these indicate that high concentration of biodegradable waste is present in the water, which requires more oxygen for decomposition by microorganisms, thus reducing the amount of oxygen dissolved in the water. This reduced dissolved oxygen has the potential to cause problems for other animals living in the water body. Concentrations of certain organisms such as fecal coliform bacteria is also monitored and controlled, so as to avoid the spread of water borne diseases.

When the sewage water enters a sewage treatment plant, it first goes through a screen which traps other unnecessary solid waste such as plastic bags etc. Such things are often found in the sewage stream because of reasons like people throwing trash into open drains, mixing of sewage and stormwater into a single drain etc. After such objects have been removed, the water is treated in multiple steps.

Initially the solid waste is segregated from the water. Then air is bubbled into the water to increase the dissolved oxygen content which helps in breaking down biological waste. Appropriate chemical dosing is done to control microbe levels and levels of other substances. On attaining the prescribed quality, the treated water is then released into another water stream.

The pumps designed for sewage pumping are different than those designed for handling drinking water or other treated water. The major difference is that sewage pumps can handle larger sizes of solid particles, and also string-like and fibrous solids which would clog the normal pumps. Sewage pump houses often have a number of high and low capacity pumps which can be operated in different combinations according to the flow rate of sewage. This flow rate also varies and is classified into peak and non-peak flows. The storage tanks and pumps are sized such that the larger pumps can handle the peak flow, and the smaller pumps are sufficiently sized to pump out the water during low flow periods quickly enough before the water quality degrades due to problems such as stagnation and growth of disease-causing organisms.

Sewage treatment plants are sometimes set up on the banks of streams/nullahs/similar water bodies into which untreated sewage is directly released. Some sort of capturing area is created from where some of the water flowing through the water body is channelled into the sewage treatment plant, treated and then released back into the same water body. This only helps in reducing the concentration of sewage related material in the water body.

Effluent treatment plant

These are mainly present in industries/other operations where chemical waste is generated and discarded through water. Several different types of waste chemicals can be generated and released into the water depending on the industry or activity taking place.

There are environmental regulations regarding acceptable levels of a number of substances in a water stream. The chemical-containing waste water is to be treated until the chemicals are at acceptable concentration levels before it can be released into any other stream of water. Unchecked release of chemicals can cause major environmental and health concerns.

Water Recycling

Though 71% of the earth’s surface is covered in water, a very tiny fraction of this is actually economically usable without requiring massive treatment. With water scarcity becoming an increasingly prominent problem, the concept of water recycling is now slowly becoming more popular. This concept essentially involves using water for one purpose, then treating it to some extent, and then reusing it for another purpose. Some examples recommended for daily household activities include reuse of non-toilet use water for flushing, and washing grains/vegetables/fruits in water kept in a container and then using this water for watering plants. The Spanish sanitaryware company Roca came up with a wash basin plus toilet designed for reuse, among some other products to enhance water conservation.

There are cases when high population density units, such as large apartment complexes, have their own sewage treatment plants in place and the treated water is used for purposes such as gardening and vehicle cleaning. It is to be noted that during summer time, some cities have made it a punishable offence to use continuously flowing tap water from pipes for gardening and vehicle washing purposes in a bid to counter the water scarcity. Economic feasibility of course plays a major role in such considerations, but if found feasible, then such in-house STPs are a good way of promoting water conservation.

An example of large-scale reuse of treated water is from Nagpur, where the treated water from the city’s main sewage treatment plant is being pumped to a nearby power plant. It is reported that around 130 to 150 million litres of treated water is supplied daily to the power plant.